if a girl that you always stared at and talked to and smiled to asked you if you would let her buy you a drink to say thank you for helping her with a problem, would you say yes?
you already have a girlfriend btw, would that even matter?
Certainly! She is expressing her gratitude and to decline the offer without good reason would be nothing short of churlish. The fact that you already have a girlfriend is totally irrelevant. If it were a bloke you probably wouldn't think twice about accepting.